1. Field of the Invention
The present invention relates to a modulator used in digital communication and the like.
2. Description of the Related Art
The FSK (Frequency-Shift Keying) is known as a conventional modulation for transmitting a digital signal through a wire or as a radio signal.
The FSK is a special case of frequency modulation. When a digital modulating signal is set at logical "0", a signal having a frequency f0 is transmitted. When the signal is set at logical "1", a signal having a frequency f1 is transmitted. This modulation is advantageous in terms of a high S/N ratio upon demodulation and a simple circuit arrangement and is used in a variety of applications.
A voltage-controlled oscillator (VCO) is used as a conventional FSK modulator for obtaining an FSK signal. More specifically, a digital modulating signal is directly or after voltage conversion input to the control terminal of the voltage-controlled oscillator. When the digital modulating signal is set at logical "0", the voltage-controlled oscillator is oscillated at the frequency f0. However, when the digital modulating signal is set at logical "1", the voltage-controlled oscillator is oscillated at the frequency f1. Under these conditions, an output signal from the voltage-controlled oscillator is used as a transmission signal without processing.
An oscillator circuit which generates low phase noise must be used as the voltage-controlled oscillator to prevent generation of noise at the frequency f1 when the voltage-controlled oscillator is operated at the frequency f0 and to prevent generation of noise at the frequency f0 when the voltage-controlled oscillator is operated at the frequency f1. For this purpose, an oscillator circuit which uses a piezoelectric device such as a surface acoustic wave (SAW) device as an oscillation source is generally used as the above oscillator circuit.
In a conventional FSK modulator having the above arrangement, in order to sufficiently increase a frequency variable width of the voltage-controlled oscillator, i.e., a difference between the frequencies f0 and f1, a resonator having a small loaded Q is required. To the contrary, in order to reduce the phase noise, a resonator having a large loaded Q is required, thus presenting two contradictory requirements. In order to assure a relatively large difference between the frequencies f0 and f1, the device such as a quartz surface acoustic wave resonator having good temperature characteristics and a large loaded Q cannot be used. In addition, when the logical level of the digital modulating signal is changed from level "0" to level "1" or vice versa, the oscillation frequency of the voltage-controlled oscillator is gradually changed from the frequency f0 to the frequency f1 or from the frequency f1 to the frequency f0 by a time constant of its oscillation loop. For this reason, when a bit rate of the input digital signal is increased, a change in frequency of the voltage-controlled oscillator cannot cope with the change in frequency.
Another FSK modulator for obtaining an FSK signal is proposed wherein an output from a first oscillator oscillated at the frequency f0 and an output from a second oscillator oscillated at the frequency f1 are switched by a switching circuit in accordance with a digital modulating signal. However, this modulator has a complex circuit arrangement as compared with the FSK modulator using the voltage-controlled oscillator. It is difficult to maintain relative precision of the oscillation frequencies of the first and second oscillators without adjustment during mass production. In addition, it is difficult to prevent interference, cross modulation, and locked oscillation between oscillators when a plurality of high-frequency oscillators are arranged on a single printed circuit board or semiconductor integrated circuit substrate. Therefore, the above-mentioned FSK modulator cannot be realized in practice.
The operation characteristics of the conventional FSK modulator using the voltage-controlled oscillator depend on characteristics of a piezoelectric device used in an oscillator circuit. A difference in output frequency when the digital modulating signal is at level "0" and at level "1", and temperature characteristics of the resonator cannot be arbitrarily selected. In addition, this FSK modulator cannot be used when a bit rate of the digital modulating signal is high.
When a plurality of oscillators and a switching circuit for switching output signals from these oscillators are used, the overall circuit arrangement becomes bulky. In addition, the difference between the output frequencies cannot be maintained to be constant without adjustment during mass production, and cross modulation and locked oscillation between the oscillators occur. Therefore, it is not easy to realize a practical circuit arrangement.